This invention relates generally to the art of making Portland-type cement and in particular to improvements in high-strength Portland-type boron-containing cements through use of mineralizers having a halogen-containing component. The composition according to the invention achieves the formation of a Portland-type cement possessing unusually high full term as well as early strengths, and acceptable setting time characteristics.
In the typical commercial production of Portland-type cements, a calcareous type material, such as limestone, and an argillaceous type material, such as clay, are used to obtain a mixture of lime, aluminum oxide, silicon dioxide, and ferric oxide. These "raw" materials are first pulverized into a homogeneous mixture, either in dry or slurry form, and then burned in a kiln, usually of the rotary type, at temperatures normally ranging from 2,600-2,800.degree. F. to form solid "clinker." The clinker is in turn ground with gypsum to form a fine-powdered cement. Certain "mineralizers" may be added to the raw mix prior to "clinkering" and certain "additives" may be added to the clinker during grinding to improve the strength and setting properties of the resulting cement. The composition of the cement depends upon the nature and proportion of the raw materials, mineralizers, and additives employed, as well as the temperature of the ignition and extent of grinding. The basic process reaction is such that the lime (usually as CaCO.sub.3) upon heating releases carbon dioxide to form CaO or free lime which in turn reacts with the alumina (Al.sub.2 O.sub.3), iron oxide (Fe.sub.2 O.sub.3), and silicon dioxide (SiO.sub.2) to form the basic components of cements.
In general, the basic components of Portland type cement consist of calcium silicates, calcium aluminates, and calcium alumino-ferrites, all of which form the hydraulically settable ingredients of such cement. The calcium silicates are the major components of Portland-type cement compositions and are present in various forms depending upon the nature of the raw mix and account in substantial part for the strength and setting properties of such cements. In the absence of a boron-containing component, tricalcium silicate (Alite) and .beta.-dicalcium silicate (Belite) are formed and stabilized. However, in boron-containing Portland-type cements, typified by U.S. Pat. No. 3,861,928 to Slater and Hamilton, issued Jan. 21, 1975, the disclosure of which is incorporated herein by reference, alpha-prime dicalcium silicate (.alpha.'-C.sub.2 S) is formed and stabilized to the exclusion of the C.sub.3 S and .beta.'-C.sub.2 S compositions which would otherwise be formed and stabilized (C.sub.3 S and .beta.'-C.sub.2 S are stabilized in such small amounts, if at all, that they cannot readily be identified by X-ray diffraction analysis). The boron-containing cements of Slater and Hamilton are also characterized by a free lime content less than about 2% and the presence of borate (as B.sub.2 O.sub.3) dissolved with lime in the .alpha.'-C.sub.2 S phase in a ratio of five moles of CaO per mole of B.sub.2 O.sub.3. The cements disclosed by Slater and Hamilton contain a relatively small portion of a boron-containing compound, such as boric oxide (B.sub.2 O.sub.3), which is added as a mineralizer to the raw mixture prior to clinkering. The addition of the boron component permits the formation of a Portland-type clinker at temperatures substantially lower (2350-2550.degree. F.) than those formerly necessary in commercial practice at about the same kiln retention time. Moreover, boron-containing cement compositions can be made to achieve full term (i.e., after 28 days) compressive strengths in the order of about 9,000 psi which is far superior to other Portland-type cements. These advantages of prior boron-containing cements have been attributed to the .alpha.'-C.sub.2 S, which is usually present in an amount of about 65 percent to about 85 percent by weight of the composition.
While boron-containing Portland-type cements typified by those of Slater and Hamilton represent a significant advance in the field of cement chemistry, their commercial use presents several drawbacks. For example, although superior full terms strengths are achieved, the results have been inconsistent and the setting times erratic. Moreover, the early and intermediate strengths for these cements are not satisfactory for many commercial purposes.
Many attempts have been made to improve boron-containing Portland cements such as to overcome the early and intermediate strength drawbacks while maintaining the excellent full term strengths. It is known that the early strength characteristics of conventional Portland-type cements is attributed to the formation of about 50-60% C.sub.3 S in the clinker, see Lea, The Chemistry of Cement and Concrete, (3d Ed. Chemists Publishing Co. 1977, pg. 82). But, to pursue the idea of increasing early strength in boron-containing cements by forming such amounts of C.sub.3 S was antithetical in view of the work of Slater and Hamilton. They had attributed the high full term strength to the formation of .alpha.'-C.sub.2 S to the exclusion of C.sub.3 S. Moreover, prior teaching in the art and belief in the industry was that C.sub.3 S could not be formed in the presence of borates. See Mircea, "Decomposition of Tricalcium Silicate With Boron Oxide," Silikatz (Ceskoslovenska Akademie Ved), 9(1) 34-42 (1965), which discloses that boric oxides react with tricalcium silicate to form free lime and a saturated solid solution of dicalcium silicate and pentacalcium borosilicate (5CaO:1B.sub.2 O.sub.3 :1SiO.sub.2).
Initial efforts to improve the properties of boron-containing Portland-type cements were directed toward development of high early strengths by means which would retain the .alpha.'-C.sub.2 S formation characteristics. Unsuccessful efforts to gain early strength included reducing the clinker particle size and varying the raw mix constituents--particularly as to the composition of the iron phase. Attempts were also made without success to gain high early strengths and more consistent setting times by use of strength accelerators and water reducing additives. Other efforts included the physical blending of high early strength cements (see U.S. Pat. No. 4,036,657 to Mehta) with the .alpha.'-C.sub.2 S-containing boron cement of Slater and Hamilton. Samples made utilizing the Metha cement (i.e., containing C.sub.4 A.sub.3 S), which has an early strength superior to the C.sub.3 S-containing cement, mixed with the boron-containing cement both before and after clinkering, exhibited low intermediate and full term strengths, e.g., 1,475 psi 7-day, and 4,150 psi 28-day strength.
Notwithstanding prior teachings and beliefs, the present invention is based upon the combined formation of C.sub.3 S and .alpha.'-C.sub.2 S in boron-containing Portland-type cements to achieve not only improved, but surprisingly superior early strengths as compared to those previously attained. In addition, it has been found that these superior early strengths are achieved within acceptable setting times and without affecting the excellent full term strengths of boron-containing cements. Moreover, the full term strengths are now attained on a more consistent basis.